1. Field of the Invention
This invention relates generally to sound systems for use with television receivers and, more particularly, is directed to sound detecting apparatus of the intercarrier type for use with television receivers.
2. Description of the Prior Art
Various types of sound detecting apparatus for use with television receivers are well known in the art. For example, in a split-carrier sound detecting apparatus, the video signal and sound signal are independently processed. However, with such apparatus, distortion of the sound output as a result of fluctuation of the local oscillation frequency may result. Further, with such apparatus, radiation jamming from the power source and deflection system may result. Further, because of the complicated circuit construction with a split-carrier sound detecting apparatus, the cost thereof is greatly increased over other sound detecting apparatus which utilize relatively simple circuit constructions. Accordingly, it is generally desirable to utilize other sound detecting apparatus, for example, an intercarrier sound detecting apparatus, in order to enjoy good sound in response to a sound-multiplexed broadcast.
Intercarrier sound detecting apparatus for use with television receivers are also well known in the art. In one known intercarrier sound detecting apparatus, the television broadcast signal is received by an antenna and passes through a tuner, a filter for extracting a video intermediate frequency (VIF) signal, and a plurality, for example, three VIF amplifier stages. The output signal from the last VIF amplifier stage is supplied to a diode envelope detector circuit which provides a beat signal having a frequency of 4.5 MHz and which is based upon the sound carrier component having a frequency of 54.25 MHz and the video carrier component having a frequency of 58.75 MHz. The beat signal from the 4.5 MHz diode envelope detector circuit is then supplied to a band pass filter which passes only the 4.5 MHz signal to a sound IF amplifier, an amplitude limiter and a frequency discriminator which produces the sound signal at an output thereof. In other words, with the aforementioned intercarrier sound detecting apparatus, the 4.5 MHz beat or difference signal is provided in response to the video carrier and sound carrier components and is then frequency-discriminated.
However, with the aforementioned circuit, because the 4.5 MHz beat signal is produced in response to the video carrier and sound carrier components, a buzz and a buzz beat is caused by the amplitude component of the video carrier component. Thus, in response to a sound-multiplexed broadcast, the intercarrier sound detecting apparatus described above generates an undesirable buzz and a buzz beat in a sound sub-channel.